In recent years, liquid crystal display devices are used as display screens, being accompanied with thinning of monitors for television receivers, personal computers or the like, and furthermore the television receivers or the like have a tendency of enlarging. Cold cathode ray tubes are generally used as the backlights for the liquid crystal display devices, but recently, it has been studied to use semiconductor light emitting devices (hereinafter referred to as LEDs) which can be operated with a low drive voltage. In this case, as shown in FIG. 11, a backlight is formed by introducing light of LEDs 112 or the like from sides of a light guide plate 111 and by taking out the light from a surface side of a light reflection surface provided on a back surface of a light reflection plate (cf. for example, Japanese Patent Application Laid-Open No. HEI7-320514), or, is formed, as shown in FIG. 12A and 12B, by arranging the light emitting devices 112 (chips or LED products) provided on the light reflection plate, so that intensity (brightness) of the light in a surface at a position of a light diffusion member 114 is uniformed by spontaneous diffusion of the light, and that the light of uniform brightness is irradiated from the surface of the light diffusion member 114.
In the former case, although brightness in almost entire surface can be uniformed by the light guide plate in a liquid crystal panel of a small type having a side of, for example, approximately 50 mm or less, a light source of a large type can not be obtained without arranging a number of the light guide plates because uniformity in a surface can not be achieved perfectly in the liquid crystal display devices having a screen larger than that described above. Here, as the light should be introduced from a side to the light guide plate 111, a light source of a large type and uniform brightness can not be obtained by arranging light guide plates (ununiformity of brightness in a surface occurs at a part of the LEDs 112), because a part of the light sources (LEDs 112) makes obstruction. Namely, a surface light source of a desired size can not be obtained by arranging (tiling) a number of light guide plates. On the other hand, being accompanied with enlarging television receivers of a thin type, enlarging of backlights for liquid crystal display devices is indispensable.
A constitution in the latter case has been employed for backlights of recent liquid crystal television receivers of a large type or the like, because a surface light source of a desired size can be obtained by arranging LEDs up to a number for making a necessary size. On the contrary, since the light from each of LEDs is irradiated to a liquid crystal panel directly through the light diffusion member 114, a distance d from a bottom surface of the LEDs 112 to the light diffusion member 114 should be approximately 70 mm in order to uniform brightness between adjacent LEDs 112, because if not, brightness at a middle part of the LEDs 112 decreases (even if the light diffusion member is provided, it eliminates directionality of light but has no function of mixing lights which the light guide plate has). Furthermore, there occurs a problem that uniform chromaticity can not be obtained, since chromaticity of each LED varies even among white LEDs and since a distance to the light diffusion member 114 should be larger because lights are not mixed perfectly in case of making white light by mixing colors by using each of LEDs of R, G, and B.
Electrically illuminated signboards used for advertisement in stations or the like have a structure, as shown for example in FIG. 13, in which several straight fluorescent lamps (hereinafter referred to as fluorescent lamps) 101 or the like are arranged in a light reflection casing (housing) 102 so as to irradiate a display panel 104 or the like through a light diffusion plate 103 or directly. The housing is formed of, for example, stainless steel, aluminum or the like in a shape of a box having an opening at a top surface side, and the fluorescent lamps 101 or the like are provided therein. Then, the housing is formed by coating a light reflection material inside so that the light from the fluorescent lamps irradiates the display panel 104 provided at a front side without being wasted (cf. for example, Japanese Patent Application Laid-Open No. 2003-330394). In case of using LEDs for the light sources, there rise same problems as that in backlights for liquid crystal display devices as described above.
As described above, in the surface light source which irradiates the light from a surface of a light guide plate by introducing the light of LEDs from a side of the light guide plate, as a part of a light source projects, arranging (tiling) a number of them can not be carried out in order to enlarge an area of the surface light source. Here, even if an upper surface of LEDs 112 are covered with a light reflection plate not to expose the light source at a surface, the brightness can not be matched with a part of the light guide plate because the portion is not a region irradiating the light. Uniform irradiation from an entire surface by diffusing the light in the light guide plate is hardly obtained because the light is apt to travel straight even if it is made easy to introduce the light into the light guide plate by using LEDs of a dome type and by narrowing directionality of the light. On the contrary, if the directionality is widened, the light can not be taken into the light guide plate because total reflection increases due to increasing of oblique incident light to the side of the light guide plate. Therefor, the light emitted from the LEDs can not be utilized efficiently, and there exists a problem that the surface light source of high brightness, uniformity and a large type can not be obtained. In LEDs of a chip type not in LEDs of a dome type, a surface light source of high brightness and a large type has not been achieved because of small amount of light flux and its indirect light.
On the other hand, in case of a right-under type in which LED chips or the like are arranged side-by-side directly at a back side of the light diffusion member 114, fine color images can not be displayed since variation of each chromaticity appears because the light of each of the LEDs 112 is irradiated directly without being mixed, and thinning can not be achieved at the same time since the LED chips and the light diffusion plate should be parted by a distance of approximately 70 mm depending on an interval between the LEDs, taking it into consideration to mix colors of R, G, B or the like. Furthermore, when the distance from the light source is large, efficiency of using the light decreases because the brightness decreases in inverse proportion to a square of the distance and, for example, LEDs of a high out put power of approximately 1 W should be arranged through an interval of 35 to 45 mm. Therefor, increasing of input power results in a large long-term change of LEDs by heat generation, necessity of heat dispersion and necessity of a space for setting a heat sink, being against to thinning, but such backlights having the heat sink (50 to 60 mm thick) has been employed in actual television receivers of a large type.
After all, there are problems that in a conventional surface light source using LEDs, a surface light source of a large type can not be obtained simply by using a light guide plate, and that in the right-under type of arranging the LEDs directly under a display plate in matrix structure, thinning the surface light source can not be achieved because a means for dispersing heat becomes necessary as output power of the LEDs is increased to make a distance from a position of the LEDs to a light diffusion member large. Further, in the right-under type, there rises still a problem that a production cost is raised significantly, since LEDs of certain brightness and chromaticity should be used by selecting the LEDs because the lights of each of the LEDs can be hardly mixed.